The present invention relates to selective passivation of the channel walls of a channelled inkjet printhead component by the chemical vapour deposition of a passivant coating and, in another aspect, to a method of vacuum processing the surface of a component in general.
1. Field of the Invention
2. Description of Related Technology
The protection of a surface by deposition of a passivant layer (e.g. Silicon Nitride) by chemical vapour deposition in a vacuum is known in the art, for example from J. Applied Physics 66, No. 6, pages 2475-2480. The technique is predominantly used in the manufacture of semiconducting devices where restriction of coating to those areas where it is needed is achieved by use of photolithographic masking. As illustrated in FIG. 1(a), a layer of masking material 1 remains at selected locations on the substrate 2 following the dissolving of areas (indicated by 3) that have not been exposed to UV radiation in a preceding step. The entire substrate is then exposed to passivant coating as indicated at 4. FIG. 1(b) shows the substrate 2 following the coating process: passivant 5 has been deposited on the areas 3 whilst any passivant deposited on the masking material has been taken away with the removal of the masking material itself. The aforementioned masking process is known in the art and works well with the manufacture of devices on planar silicon wafers.
Passivation of channelled ink jet printheads is discussed in general terms in EP-A-0 364 136, incorporated herein by reference. FIG. 2(a) is a section through a printhead of the kind disclosed in this document taken perpendicular to the longitudinal axis of the channels: such devices comprise an array of channels 12 formed in a sheet 14 of piezoelectric material, suitably lead zirconium titanate (PZT), that has been poled in its thickness direction as indicated by arrows 15. Each channel is defined by side walls 16, a bottom surface 18 and a top sheet 20 and has formed on the surface of each side wall an electrode 34.
As is known, for example from EP-A-0 277 703 incorporated herein by reference, application of a electric field across electrodes 34 formed on opposite surfaces of a side wall 16 causes the piezoelectric material of the side wall to deflect in shear mode, thereby causing the ejection of an ink droplet from a nozzle associated with the channel.
As shown in FIG. 2(b), which is a sectional view taken along the longitudinal axis of a channel, such a nozzle 24 can be located at the forward end of each channel 12 which in turn comprises a forward part 36 of uniform depth which is plated to approximately one half the channel depth and a rearward part 38 of lesser depth which is fully plated over the base and walls to form connection tracks. The forward part of the electrodes in the channel apply the aforementioned actuating electric field whilst the rearward, connection track part of the electrodes are connected, e.g. by wire bonding, to actuating voltage supply means (not shown). Nickel, nichrome (an alloy of nickel and chromium) and aluminium have proved particularly suitable as electrode materials due to their high conductivity and suitability for wire bonding.
Subsequent passivation of the electrodes on the channel walls of such a printhead is necessary to protect the electrodes from attack by the ink contained in the channels during operation of the printhead. Aluminium in particular requires passivation to inhibit electrolysis and bubble formation or corrosion which could occur if the electrode were in direct contact with the ink. Protection is particularly desirable where the ink is aqueous or otherwise electrically conductive.
The composition of the passivation layer is chosen act as an electron and/or ion and/or ink barrier and is preferably configured so as to extend down one channel side wall, across the base of the channel, up the other channel side wall and over the top of that wall into the adjacent channel, thereby creating a continuous protective layer free of any edges under which ink might otherwise seep. A chemical vapour deposition process particularly suitable for the passivation of the xe2x80x9cdeepxe2x80x9d channels shown in FIGS. 2(a) and (b)xe2x80x94i.e. channels having an aspect (height/width) ratio of at least 3:1xe2x80x94is disclosed in WO95/07820 incorporated herein by reference.
It will be appreciated that the connection track in the rearward part 38 of the printhead must be kept free of passivation in order that a connection (generally a wire bond) from the track to a driving circuit can be made. However, the aforementioned photolithographic masking techniques conventionally used when depositing a passivant layer by chemical vapour deposition have proved difficult to use in such a situation: in particular, application and removal of masking material on the walls and bottom surface of the rearward part of each channel (typically having a width of 60-90 xcexcm and a depth of 20-25 xcexcm) has proved complex and difficult. These problems are exacerbated where it is desired to mask not only the rearward part 38 but also a section of the full-depth, forward part 36 of the channel (typical width 60-90 xcexcm, typical depth 300-400 xcexcm), as proposed for the printhead constructions disclosed in co-pending International application no. PCT/GB97/01083 (WO97/39897, belonging to the present applicant and incorporated herein by reference.
The present invention has as one objective a process for the selective passivation of the walls of the channel of a deep channel inkjet printhead that does not share the disadvantages of the conventional techniques and yet guarantees accurate placement of the passivation coating in the inkjet printhead channel.
Accordingly, the present invention comprises in a first aspect in a process for the selective passivation of the channel walls of a channelled inkjet printhead component by the chemical vapour deposition of a passivant coating, the process comprising mounting said component in a support in registration with a datum location therein, the support having masking means for masking selected areas of the component, and depositing the passivant coating on unmasked portions of the channel walls.
By virtue of the mask, fixed to the support rather than being located on the component itself, it is possible to shield from passivation in a simple but effective manner certain sections of the channel walls of the channels formed in the component. Furthermore, accuracy of masking is guaranteed by a datum location on the support that allows the component to be passivated to be accurately positioned relative to the support and thus relative to the mask.
Advantageously, the masking means are integral with the support. The inventors have found that chemical vapour deposition in accordance with the present invention generally results in the heating of the surface of the mask, and such an integral construction facilitates heat flow away from the surface into the remainder of the support, thereby reducing any tendency the mask might have to warp out of alignment with the component being passivated.
Preferably, a first, channelled surface of the component is held resiliently in abutment with the support. It has been found that this feature is particularly useful where the component to be processed comprises a materialxe2x80x94such as lead zirconium titanate (PZTxe2x80x94having a dimensional tolerance lower than that which can be achieved with conventional materials such as silicon and where there may be a significant variation in thickness across the component. In such a situation, conventional clamping on both surfaces of the component would give rise to a variation in the clamping forces and distortion of the component. Abutment of only one surface of the component against the support avoids such problems.
That surface of the component which opposes the first, channelled surface of the component advantageously undergoes heat transfer with a fluid. Such direct transfer, without the intervention of a base plate and associated heat sink compounds and pads, allows more precise control of the temperature of the component.
A membrane may be interposed between the component and the fluid. Whilst such a membrane presents little resistance to heat transfer, it does maintain the vacuum in the chamber in the event that the component is or becomes porous (e.g. by cracking). Furthermore, the membrane is advantageously attached to the support so as to retain the component within the support, if not in complete abutment with the support, resulting in an assembly that is more easily handled.
A second aspect of the invention comprises a method of vacuum processing a first surface of a component in a vacuum chamber housing a component support, wherein a membrane separates a second surface of the component which opposes said first surface from a heating/cooling fluid, heat transfer taking place between the component and the fluid across the membrane, the method comprising the steps of placing the component in a support such that the first surface of the component abuts the support, attaching the membrane to the support so as to retain the component in the support, and vacuum processing the first surface of the component so located in the support.
The arrangement of a support which abuts the side of the component to be processed and to which a membrane is attached to retain the component in the support allows the non-processed side of the component to be cooled (or even heated) by a fluid whilst ensuring that, in the event of the component being or becoming porous (e.g. by cracking), none of the fluid will escape into the vacuum chamber (which would disrupt the vacuum necessary for processing). Furthermore, by retaining the component in the support by the membrane, an assembly is created which is easier to handle than the component alone or the component sitting loose in the support.
The aforementioned arrangement is of particular advantage where the component forms part of a wafer. Such wafers can be difficult to handle, especially wafers of piezoelectrically-active material and lead zirconium titanate (PZT) in particular.
Further advantageous embodiments of the present invention are set out in the accompanying claims, description and figures.